The expected sex ratio for a given population of mammals is approximately 50% males. Attempts to alter this ratio by genetic selection in mice and fruit flies have been unsuccessful (see, e.g., Falconer, Am. Nat. 88:385 (1954); Laurie, Proc. R. Soc. London 133:248 (1946)). However, alterations of the sex ratio have been described in several wild and laboratory populations of mammals. Sex ratio alterations have been ascribed to a number of environmental factors including food supply (see, e.g., Austad and Sunquist, Nature 324:58 (1986); McClure, Science 211:1058 (1981); Meikle and Drickamer, J. Repro. Fert. 78:587 (1986)); timing of mating (see, e.g., Hendricks and McClintock, Physiol. Behav. 48:625 (1990); Horning and McClintock, Anim. Behav. 47:1224 (1994); Huck et al., Behav. Ecol. Sociobiol. 26:99 (1990)); social dominance status (see, e.g., Meikle et al., Anim. Behav. 46:79 (1993)); and intrauterine position of the fetus (see, e.g., Clark et al., Nature 364:712 (1993); Vandenbergh and Huggett, PNAS (USA) 91:11055 (1994); Clark and Galef, Physiol. Behav. 57:297 (1995)).
Further, some alterations in sex ratio have been made in domestic farm animals by taking advantage of differential characteristics of the Y- and X-bearing sperm. To date, no controlled physiological manipulation of the pregnant female has been reported to alter sex ratio in naturally conceived mammalian populations.